It is known, that flexible pipes with diameters larger than 50 mm are used in large quantities in several areas of industry, for example, in onshore and off-shore oil mining.
These flexible pipes must possess extremely complex properties because they have to withstand chemical and mechanical effects caused by the gases and liquids transported by these pipes. Flexible pipes must also withstand many environmental effects. For example, pipes used in off-shore oil mining must withstand the hydrostatic pressure of sea water, the stresses generated by ocean waves, and the impairing effects of sea water and sunshine.
During the years, two fundamentally different flexible pipe structures have been developed to withstand the above mentioned h effects. They are non-bonded and bonded flexible pipe structures.
Non-bonded flexible pipe structures typically have a hose body comprised of many different structural parts that are not permanently joined together. A widely used non-bonded structure for flexible pipes is disclosed in U.S. Pat. No. 4,549,581. The U.S. Pat. No. 4,549,581 describes a flexible pipe having an inner plastic layer, a low pitch inner steel layer, high pitch outer steel layers and an outer plastic layer, which are not permanently joined together. This construction permits the adjacent layers to move in relation to each other during the movement of the pipe. Additionally, the adjoining parts forming the coupling of the pipe, which ensure sealing, are only in contact. No chemical bonding is provided between them.
Non-bonded flexible pipe structures (having plastic, liquid and/or gas barrier layers or inner liners) can be advantageously manufactured in several kilometer lengths, without using intermediate couplings. Additionally, the plastic inner liner makes these pipes especially suitable for transporting gases or mixtures of liquids and gases.
Unfortunately, the semi-crystalline plastic inner liner and cover of non-bonded pipe structures tend to elude stress. This results in creep that increases due to high speed flow and temperature. Creep may cause the failure of the sealing, which leads to the destruction of the pipe.
Another disadvantage of flexible pipes with non-bonded structures is that the annular space enclosed by the inner liner and cover is liquid permeable. Therefore, in case of damage to the cover, inpouring sea water will cause the corrosion of metallic parts in an annular space enclosed by the inner liner and the cover. Still another disadvantage of non-bonded structures is that the plastic inner liner is not connected permanently to adjacent structural parts, which may result in out sliding or lengthening and the wrinkling of the inner liner due the high temperature, pressure and speed of flow of the transported medium. This in turn, can lead to the destruction of the flexible pipe.
The above advantages and disadvantages apply not only to the hoses described in U.S. Pat. No. 4,549,581, but generally to all non-bonded flexible pipes structures.
Bonded flexible pipe structures typically have a hose body comprised of many different structural parts that are permanently joined together. More specifically, the structural parts between the inner liner and the outer barrier layer (cover) of the flexible pipe are permanently joined or connected so that the parts can not be separated with nondestructive methods.
Bonded flexible pipe structures generally include reinforcements in the form of steel cables or wires and textiles. The inner and outer barrier layers (inner liner and cover) and the layers which fasten the different parts are made of elastomers. The permanent connection between the parts is created by forming a network.
A widely used bonded structure for flexible pipes is disclosed in HU Patent 183,563. The flexible pipe structures disclosed in this patent have been designed for the delivery of gases or mixtures of liquids and gases. In each of these structures, the first member in the inside of the flexible pipe, from the inside toward the outside, is a flexible stripwound metal pipe (e.g. steel stripwound). This flexible stripwound metal pipe serves as a protective layer against the effects of gas diffusion, such as the blistering of the elastomer inner liner. This also means that an additional member, such as a flexible stripwound metal pipe, must be built into the flexible pipes manufactured with elastomer inner liners, which are used for the delivery of gases or mixtures of liquids and gases. These additional members provide protection against the blistering and delamination of the inner liner.
One of the advantages of bonded flexible pipe structures is that the reinforcing members in the wall structure are sealed from the corrosive media. Another advantage is that the structural parts of the pipe structure are joined firmly, therefore, undesirable creeping effects can not appear. This makes forming a reliable and persistent sealing possible between the hose body of the flexible pipe and the couplings at its ends. A further advantage of this structure is that the high temperature, pressure, and speed of flow of the transported medium cannot detach the parts of the hose structure (especially the inner liner) from their surroundings and scrape them off.
However, the elastomer-based inner liners of bonded flexible pipe structures are less able to resist the harmful effects caused by the flow of the transported high pressure and high velocity gases or mixtures of liquids and gases, than inner liners made of thermoplastic to materials. This is particularly true in cases when sudden pressure decreases frequently occur during their utilization.
At the present level of technology there are no known flexible pipes comprised of an inner liner made of thermoplastic material, that have partially or fully connected joining members that can not be broken by nondestructive methods, and that have a sealed area between the hose body and the couplings that is maintained even if creeping occurs on the section of the semi-crystalline structured inner liner which is in this sealed area.
The authoritative API 17J and ISO 13628-2 standards only apply to thermoplastic based flexible pipes with non-bonded structures.
The API 17K standard, which is currently in preparation will contain requirements for the rubber-based flexible pipes with bonded structures.
There are no known flexible pipes, which are fitted with coupling, that have a bonded structure and are manufactured with a thermoplastic gas barrier layer.
The object of the present invention is, therefore, to develop a flexible pipe comprising a thermoplastic inner liner, and if necessary, a flexible stripwound metal pipe in order to overcome the disadvantages of the presently known flexible pipe structures, wherein the permanent connection between the structural parts of the flexible pipe, the coupling and the thermoplastic inner liner is established by the network forming of an elastomer.